(1) Field of the Invention
The present invention relates to single-crystal diamond substrates having a large area, and methods for producing such substrates.
(2) Description of the Related Art
Diamond, which exhibits outstanding properties as a semiconductor, is a promising material for use in semiconductor devices, such as high-output power devices, high-frequency devices, light-receiving devices, etc. In particular, in order to realize the practical use of diamond as a semiconductor material, wafers of single-crystal diamond having a large area and uniform quality are required.
Typical methods for growing single-crystal diamond heretofore used include a high-pressure synthesis method and a vapor-phase synthesis method. Of these methods, the high-pressure synthesis method can produce substrates with an area of only up to about 1×1 cm, and cannot be expected to produce single-crystal substrates with a larger area. Furthermore, single-crystal diamond substrates with an area of about 5×5 mm or more are not readily available, nor is it easy to increase the substrate area of these substrates.
For this reason, a method for producing a so-called mosaic diamond has been developed to prepare a single-crystal diamond with a large area. This method involves growing diamond crystals by a vapor-phase method on a plurality of diamond crystals aligned on a support surface, and bonding the aligned diamond crystals, thereby producing a large diamond crystal (see Meguro, Nishibayashi, and Imai, SEI Technical Review 163, 53 (2003)).
In the known production method of a mosaic diamond, only single-crystal diamond substrates are used, or single-crystal diamond substrates are used together with polycrystal diamonds or other material(s), as the substrates to be bonded. In either case, these diamond substrates are bonded by growing a diamond thereon by a vapor-phase method.
Among these methods, as one example of a method for producing a large single-crystal diamond by using only single-crystal diamond substrates, and bonding these substrates, a method for producing a large diamond crystal has been reported, in which the spacing and differences in height among the single-crystal diamond substrates to be bonded are adjusted to be within predetermined ranges, and a diamond crystal layer is grown on these substrates by a vapor-phase method, thereby suppressing the growth of non-epitaxial crystallites along the boundary between substrates (see Japanese Unexamined Patent Publication No. 7-48198).
Another method has been proposed in which diamond substrates having suitable off-angles and off-directions are selected and aligned; subsequently, diamond crystals are grown by a vapor-phase synthesis method preferentially in the direction of adjacent single crystals to promote bonding (see Japanese Unexamined Patent Publication No. 2006-306701).
Also known are a method in which cleaved faces are used as the side faces to be bonded, and a method in which the side faces to be bonded are angled (see Japanese Examined Patent Publication No. 6-53638 and EP 0687747 A1).
It is noted that methods for growing single-crystal diamond on diamond substrates by homoepitaxial growth using a vapor-phase synthesis method are applied to, for example, the synthesis of semiconductor-grade, high-quality diamond. However, during epitaxial growth of diamond by a vapor-phase synthesis method, many defects such as non-epitaxial crystallites and hillocks tend to occur, making the synthesis of a large single-crystal diamond difficult.
The formation of these defects strongly depends on the off-angle and off-direction of the substrate surface on which a diamond is grown. It has been reported that even a change of 1° in the off-angle and off-direction of this substrate surface will alter the properties of the growth layer (see, e.g., H. Okushi, Diamond and Related Materials 10 (2001), 281-288; and O. Maida, H. Miyatake, T. Teraji, and T. Ito, Diamond and Related Materials 17 (2008), 435-439.). The off-angle and off-direction dependencies also vary according to the synthesis conditions. Thus, the properties of the growth layers do not become uniform, unless the synthesis conditions are adjusted according to the off-angle and off-direction of each substrate during the growth of single-crystal diamond on substrates having different off-angles and off-directions.
Defects on the substrate surface are also continued in the growth layer, and the positions of these defects cannot be controlled for each substrate. Therefore, the presence of these defects is one hindrance to achieving uniform growth layer properties. Furthermore, it is known that the properties of a growth layer are also affected by strains present in the substrate prior to growth (see P. S. Weiser and S. Prawer, K. W. Nugent, A. A. Bettiol, L. I. Kostidis, D. N. Jamieson, Diamond and Related Materials 5 (1996), 272-275.).
Generally, in methods for preparing mosaic diamonds by growing diamond crystals on a plurality of diamond crystals by a vapor-phase method, the threshold at which the off-angles of diamond substrates to be bonded are considered to be identical is 1° or more at a minimum. However, even a difference of 1° in off-angle will result in variations in the qualities of growth layers under identical conditions. On the mosaic substrate bonded by this method, a single-crystal layer having a different quality according to each crystal region of the bonded single crystals will grow. Similar problems also arise in the method described above, wherein substrates with different off-angles and off-directions are positively used and bonded with one another to produce a mosaic substrate (see Japanese Unexamined Patent Publication No. 2006-306701).
As stated previously, in homoepitaxial growth on diamond substrates using a vapor-phase synthesis method, the growth layer is affected not only by the off-angle, but also by the off-direction and substrate properties such as strains and defects in the substrate. Nevertheless, none of the previously known methods for producing mosaic substrates have proposed an effective solution to make the properties of substrates to be bonded uniform.
Furthermore, when diamond is used as a material for semiconductor devices, impurities are intentionally doped into the growth diamond on substrates (diamond wafers). It is known that the amount of the impurities doped into the growth layer, as well as the resulting change in crystallinity, will depend on the substrate properties (see K. Arima, H. Miyatake, T. Teraji, and T. Ito, Journal of Crystal Growth 309 (2007), 145-152.). Therefore, if large diamond wafers obtained using any of the above-mentioned methods do not have a uniform off-angle, off-direction, strain distribution, defect distribution, etc., it is expected that the devices prepared thereon will also exhibit non-uniform properties. Therefore, the use of these mosaic wafers with non-uniform properties will obviously result in an extremely low yield of devices that can withstand practical use. Further, because mosaic diamond substrates must be strong enough to withstand processing for device preparation, it is sometimes necessary to additionally grow a single-crystal diamond onto the bonded substrates. If the single-crystal diamonds used as the substrates to be bonded have different properties, it will be difficult to uniformly grow the single-crystal diamond on the bonded substrates.
Furthermore, as mentioned above, it is difficult to obtain a desired number of crystals whose substrate properties are uniform. For example, the preparation of diamond substrates that meet predetermined requirements by processing a single-crystal diamond requires a great deal of time, because processing of the diamond crystal is very difficult; moreover, the preparation of a precisely processed single-crystal diamond substrate is difficult. In particular, it is impossible to impart specific strain and defect distributions to a given substrate.
For reasons as stated above, despite their high demand, single-crystal diamond substrates with a large area that can withstand practical use are not available yet.